Take sildenafil exactly as directed. Sexual activity may be a strain on your heart, especially if you have heart disease. Its effects might last longer if you have mild to moderate ED. Brand names Why is this medication prescribed? Sildenafil is in a class of medications called phosphodiesterase PDE inhibitors. Take sildenafil as needed before sexual activity. The information provided is for educational purposes only and is not intended to replace discussions with a healthcare provider.Viagra is usually taken only when needed, 30 minutes to 1 hour before sexual activity. You may take it up to 4 hours before sexual activity. Do not take Viagra more than once per . Sildenafil (Liqrev, Revatio) is used to improve the ability to exercise in adults (Liqrev, Revatio) and children 1 year of age and older (Revatio) with pulmonary arterial . VIAGRA is a prescription medicine used to treat erectile dysfunction (ED). You will not get an erection just by taking this medicine. Tell all your healthcare providers that you take .